There are a variety of packaged products, especially foodstuffs, where the packaging film or substrate is printed on one side and an adhesive applied to the opposite side to seal the package. Typically, sealing is achieved using cold seal adhesives, of which the most commonly used adhesives are acrylic based. In order to integrate the packaging and sealing process with the assembly line production of the product the packaging film is manufactured in a large roll with the printed side of the film in contact with the adhesive-coated side of the film. Left unprotected, the pressure exerted on the matching print and adhesive surfaces of the roll during storage or the shearing action of high speed separation of the surfaces during the packaging step of manufacturing results in transfer between the surfaces of ink to adhesive and/or adhesive to ink. To protect against this event, the printing ink surface is coated with a protective lacquer, a cold seal release lacquer (CSRL), that forms a barrier coating between the adhesive and print surfaces of the rolled film.
Cold Seal Release Lacquers (CSRL) were originally developed for the chocolate candy bar market. Cold Seal Technology has now expanded into the snackfood packaging industry. Cold Seal Release Lacquers are generally a polyamide or nitrocellulose/polyamide blend system designed to act as a protective coating for the printed side of a film package. The CSRL must provide gloss and scuff protection to the finished product as well as being block resistant while the printed film is in roll form. High wind-up tension within the printed roll makes the CSRL/Cohesive interface critical. If the CSRL does not provide a smooth, easy unwind, ink picking, film tearing or difficult machining will occur.
Polyamide resins have been found to be uniquely useful in CSRL applications. Besides providing the requisite release, polyamide resins confer high gloss and scuff resistance to the packaged product. Accordingly, they have remained the resin of choice for CSRL applications.
Polyamides are formed by combining carboxylic acids, mostly dibasic, with organic polyamines, usually diamines. The acid and amine groups immediately react to form a salt. Upon heating to 140.degree. C. or higher, this salt decomposes with the evolution of water to give an amide bond.
Alcohol soluble polyamides are widely used in alcohol based flexographic inks for printing on plastic film. Environmental concern over the amounts of volatile organic solvents in the atmosphere has led to a desire to use aqueous solutions that have less volatile organic solvents contained therein. In order to meet new Environmental Protection Agency regulations, it is desirable to employ water based flexographic inks with reduced levels of volatile organic solvents. To accommodate the reduced levels of volatile organic solvents, the polyamide resins used should have increased water solubility and yet retain other desirable properties of polyamide resins. One technological difficulty has been in making water dispersible polyamides which provide inks with properties such as adhesion, gloss, water resistance, and blocking resistance.
One method known in the art to achieve water dispersibility in polyamide resins is to prepare the polyamide having a high acid value (AV). When the free acid groups of the resin are neutralized with ammonia, the resin becomes water soluble. After printing, the ammonia evaporates and the resin develops water resistance.
U.S. Pat. No. 4,810,747 teaches the production of CSRL using an aqueous dispersion of polyamide. However, the polyamide resin is actually a blend of two polyamide resins of different molecular weights to provide a blend with high AV, i.e., low molecular weight, but high hardness, i.e., high molecular weight.
It is an object of the present invention to provide an aqueous cold seal release lacquer having high abrasion resistance, block resistance, and bond strength with low transfer properties and coefficient of friction.
It is a further object of the present invention to provide an aqueous CSRL having the stated properties prepared from a polyamide block copolymer.